(a) Field of the invention
The present invention relates to a finder optical system for a single-lens reflex camera.
(b) Description of the prior art
In recent years, single-lens reflex cameras are being made small in size and, in relation thereto, pentagonal roof prisms in finder optical systems are also being made small.
Generally, in finder optical systems, the point where the ray coming from the point of the highest image height intersects with the optical axis after going out from the pentagonal roof prism and being refracted by the eyepiece is the eye point of the finder optical system. When the pentagonal roof prism is made smaller, the effective areas of reflecting surfaces and light exiting surfaces thereof become small, and the height of the ray which goes out from the pentagonal roof prism becomes low. Therefore, the eye point necessarily comes to a position near the prism.
As a result, the eye relief (the distance from the eyepiece to the eye point) becomes short. A finder with a short eye relief is very inconvenient for use especially for a person who wears spectacles. In the above-mentioned case, the eye relief can be kept long to some extent when the refractive power of the eyepiece is made weak. In that case, however, the magnification of the finder becomes low, and this is not desirable.
To prevent the above-mentioned decrease in the magnification, a known finder optical system is arranged that the eyepiece thereof comprises a negative lens and a positive lens in the order from the object side, refractive powers of respective lenses are made strong so as to thereby shift the rear principal point rearward and, at the same time, the eyepiece as a whole is put to a position as close as possible to the light exiting surface of the pentagonal roof prism so that the ray enters the eyepiece at a position as high as possible, said known finder optical system being thereby arranged that the eye relief thereof is made long and, at the same time, the magnification of the finder is kept high to some extent.
However, when the refractive powers of respective lenses are made strong as described in the above, aberrations necessarily increase and, especially, it becomes difficult to correct astigmatism and distortion.